这是因为机体每小时可以吸收的水分量是一定的。
That is because the body can only absorb so much fluid per hour (about 1 liter or less).
白天蒸腾作用散失的水分超过根系吸收的水分，因而叶子的水势就下降。
During the day water loss by transpiration exceeds water absorption by the roots and leaf water potential decline.
看来，为了让种子吸收水分，种子的外壳必须裂开或破裂。
It appears that the shell has to crack open or break down somewhat to allow the seed to soak up water.
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